Nano-imprint lithography is based on embossing adapted to the needs of semiconductor processing. Nano-imprint lithography is essentially a micromolding process in which the topography of a template patterns a photoresist on a wafer. In photolithography, by contrast, the resist is patterned by optical exposure and development. Unlike photolithography, imprint lithography does not use reduction optics. Instead, the size of the template determines the size of the pattern. Thus masks for nano-imprint lithography are often referred to as 1X masks. One advantage of nano-imprint lithography is that the parameters that limit resolution in classic photolithography (including wavelength and numerical aperture) do not apply. Nano-imprint lithography resolution is primarily limited by the resolution of the template fabrication process.
One type of nano-imprint lithography is based on the ancient craft of embossing, with an adaptation to modern semiconductor needs. The technique uses a template, e.g., made of fused silica, with a circuit pattern etched into a raised portion of the template referred to as a mesa. The pattern may be etched into the surface of the mesa using an e-beam writer. A pattern of drops of a low viscosity, silicon-containing monomer is deposited on a substrate (e.g., a semiconductor wafer). The patterned surface of the template is gently pressed into the monomer drops on the substrate. The monomer fluid fills spaces in the pattern etched in the template. The monomer is then polymerized into a hard material, e.g., by exposure to ultraviolet (UV) radiation. The patterned surface of the template may be covered with a release layer to facilitate separation of the template from the hardened polymer. Upon separation of the template, the circuit pattern is left on the surface. A residual layer of polymer between features in the pattern may removed by an etch process, and a replica of the pattern may then be used in semiconductor processing for etch or deposition. It is possible to make features as small as 20 nm with this technique.
One problem with the above technique occurs as a result of non-uniform spreading of the monomer drops after contact with the patterned surface of the mesa. The drops are typically deposited using a computer controlled dispenser similar to an ink-jet printer. The localized spreading of the drops is partly dependent on the density of features in the circuit pattern oil the mesa. The pattern of monomer drops often must be adjusted manually to compensate for variations in feature density. Unfortunately, such manual adjustment is difficult and slow, which increases cost and reduces yield.
Thus, there is a need in the art, for a method of compensating for variations in the drop spreading that overcomes the above disadvantages.